The invention relates more particularly to a boosted brake for a motor vehicle, of the type which comprises a pneumatic brake booster actuating a master cylinder, of the type in which the booster comprises a rigid casing inside which can move a transverse partition sealably delimiting a front chamber subjected to a first pressure and a rear chamber subjected to a second pressure varying between the first pressure and a pressure greater than the first pressure, of the type in which the booster comprises a moving piston fixed to the moving partition and comprising a front face which can act on a primary piston of the master cylinder by way of a reaction disk housed in a cage interposed between the moving piston and the primary piston, of the type in which the booster comprises a control rod which moves in the piston selectively as a function of an axial input force exerted forward against a return force exerted on the rod by a return spring, of the type in which the booster comprises a plunger which is arranged in front of the control rod in the piston and which comprises at its rear end at least one annular rear seat of a three-way valve which can move progressively between a position in which, with the control rod at rest, the front chamber and the rear chamber are in communication, and a position in which, with the control rod actuated, the second pressure prevailing in the rear chamber increases, the valve placing the rear chamber in communication with the pressure which is greater than the first pressure, and of the type in which the booster comprises a feeler, arranged at the front end of the plunger and passing through a bore leading from the piston, which, when the control rod is in the rest position, is arranged at a defined jump distance from the reaction disk and which is able, when the control rod is actuated with an input force whose intensity is greater than a first defined intensity, to come into contact with the reaction disk in such a way as to transmit to the plunger and to the control rod the reaction force of the master cylinder, the ratio of the area of the reaction disk in contact with the cage to the area of the feeler in contact with the reaction disk defining a first defined boost ratio, and of the type in which the cage comprises at least one moving decompression wall which, when the control rod is actuated with an input force whose intensity is greater than a second defined intensity greater than the first, is able to move so as to create in the cage an additional volume in which a front part of the reaction disk is able to expand in order to reduce the reaction force transmitted to the feeler by way of the rear face of the reaction disk, the ratio of the area of the reaction disk in contact with the cage to the area of the feeler in contact with the reaction disk defining a second boost ratio, which is greater than the first.
Many examples of boosted brakes of this type are known.
In such a boosted brake, the moving decompression wall generally forms part of a device which is attached to the cage and which is interposed between the reaction disk and the primary piston of the master cylinder.
This device conventionally consists of a housing having one face attached to the reaction disk, this face being drilled to allow the passage of a cylindrical decompression piston which, for its part, is also arranged in contact with the reaction disk substantially within the axis of the feeler. The decompression piston is returned elastically toward the reaction disk by a helical spring which is housed inside the housing and which has substantially the same diameter as the decompression piston. When the intensity of the input force exceeds the second defined value, the decompression piston is pushed back in the housing, compressing the helical spring, and it thus creates a free volume which allows the reaction disk to be decompressed.